Prolactin is a pituitary hormone that, when elevated in men, can suppress GnRH and cause secondary hypogonadism. High prolactin can indicate a pituitary adenoma (prolactinoma), medication side effects (especially antipsychotics and SSRIs), or hypothyroidism. Symptoms of hyperprolactinemia include low libido, erectile dysfunction, fatigue, and in rare cases, galactorrhea.
Baseline prolactin testing helps rule out prolactinoma as a treatable cause of low testosterone.